[e.cash] Improve network upgrade banner

Summary:
The countdown for the network upgrade banner needed improvement. Im adding it as a seperate component here
It is following the same logic as the countdown on avalanche.cash, and using the avalanche.cash API

The basic idea is it fetches the api on load

• if it fails it doesnt load the banner
• if we get a response it sets the timestamp for the countdown, and get the blocksUntilUpgrade
• once the timestamp is reached there is another method to start polling the API every 60s to get the blocksUntilUpgrade
• Once this value is < 0 the network upgrade is considerd finished, the final text is shown and the polling stops

Test Plan:
preview the site and check it looks good

testing the actual functionality is a bit more invloved as not sure I have time to add tests, but it can be done manually
by running npm run dev

• add const TESTING_TIMESTAMP = Date.now() + 10000; somewhere
• replace setTimestamp(data.upgradeTimeStamp * 1000) with setTimestamp(TESTING_TIMESTAMP); in the useEffect
• replace if ( newBlocksUntilUpgrade !== blocksUntilUpgrade && newBlocksUntilUpgrade !== null ) { setBlocksUntilUpgrade(newBlocksUntilUpgrade); }

with setBlocksUntilUpgrade(blocksUntilUpgrade - 1); in the polling useEffect

• update the polling interval to a lower number if you like

This should run through every stage of the countdown and stop polling once its complete

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14786